This invention relates to a balanced modulator used at a mixer stage of the front end portion of a signal receiver. An object of the invention is to provide a double balanced type balanced modulator which generates less cross modulation distortion and inter modulation distortion correspondingly to an input signal having a wide level range at a signal receiver, and has low noise characteristics with respect to a small input signal level.
FIG. 1 shows a generally well-known double balanced modulation circuit, in which transistors Q.sub.1 -Q.sub.2, and Q.sub.3 -Q.sub.4 and Q.sub.5 -Q.sub.6, are emitter coupled transistor pairs and constitute a basic unit of the double balanced modulator. Transistors Q.sub.7 -Q.sub.8 are constant current transistors with respect to transistors Q.sub.5 -Q.sub.6 and the current values of transistors Q.sub.7 -Q.sub.8 are determined from the current value through a diode D.sub.1 used for a current mirror. Resistances R.sub.1, R.sub.2 and R.sub.3 serve to further stabilize operation of the current mirror. Input terminals 1-2 are differential input terminals for local oscillation signal; terminals 3-4 are differential input terminals for a received signal, terminal 5 is a connecting terminal for an external resistance and power source in order to determine the bias current at differential transistor pair Q.sub.5 -Q.sub.6 ; terminal 6 is a ground terminal; terminals 7-8 are differential output terminals of the balanced modulation circuit; terminals 9-10 are external resistance connecting terminals for adjusting the frequency conversion conductance of the balanced modulation circuit. Principal characteristics of the balanced modulation circuit in FIG. 1 are represented by frequency conversion conductance, noise figure and maximum acceptable signal input, which are determined by the value of external resistance connected to terminals 9-10, the bias current values of differential transistor pair Q.sub.5 -Q.sub.6, and the level of the local oscillator signal applied to terminals 1-2. Namely, the value of external resistance connected between terminals 9-10 is represented by R.sub.E, frequency conversion conductance gm is given substantially by the following equation: EQU gm=k(1/R.sub.E) (1),
where k is a constant dependent on a local signal level applied to differential input terminals 1-2 for a local oscillator signal, and converges into a constant value equal to 4/.pi. if the injection signal level is of a sufficiently large value. The noise figure, which is affected by the local signal level, the external resistance connected between the terminals 9 and 10, and the signal source impedance of received signal connected to terminals 3-4, is greatly affected by bias currents in the differential transistor pair Q.sub.5 -Q.sub.6 so that the noise figure, as is well known, usually deteriorates when a large bias current is adopted. Furthermore, the level of maximum acceptable signal input to the balanced modulation circuit is determined by bias current in differential transistor pair Q.sub.5 -Q.sub.6, and the external resistance connected between the terminals 9 and 10. When differential transistor pair Q.sub.5 -Q.sub.6 is operatable in the condition of being out of a non-linear zone of saturation or cut-off, a value of amplitude V.sub.smax of signal input level, i.e., the maximum acceptable signal input level, applied to terminals 3-4, is given by EQU V.sub.smax =R.sub.E .multidot.I.sub.B ( 2),
where R.sub.E is the same as in Equation (1), I.sub.B represents the bias current values in differential transistor pair Q.sub.5 and Q.sub.6, equal bias currents I.sub.B being assumed to flow in transistors Q.sub.5 and Q.sub.6.
Thus, the principal characteristics of the double balanced modulation circuit, when used as the mixer at the front end portion of a signal receiver, are as described in the above.
It is a desire to improve the sensitivity of the mixer at the front end portion of signal receiver by means of a larger frequency conversion conductance and a low noise figure, and to reduce cross modulation distortion and inter modulation distortion by keeping the maximum acceptable signal input as large as possible. Upon review of the above-noted characteristics of the balanced modulation circuit, the above-noted desired performance is seen to be difficult to fully meet. In other words, if the frequency conversion conductance is intended to be increased, R.sub.E, as seen from Equation (1), must be reduced, at which time the maximum acceptable signal input level deteriorates as seen from Equation (2). Accordingly, it is desired that bias current in Equation (2) is increased in order to increase the maximum acceptable input level without reducing the frequency conversion conductance. In this instance, a problem has been created in that the noise figure generally deteriorates.